Because of the interest in recent years to reduce the emission of pollutants from burners used in large furnaces and boilers, significant improvements have been made in burner design. In the past, improvements in burner design were aimed primarily at improving heat distribution. However, increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants.
Oxides of nitrogen (NOx) are formed in air at high temperatures. These compounds include, but are not limited to nitrogen oxide and nitrogen dioxide. Reduction of NOx emissions is a desired goal to decrease air pollution and meet government regulations. In recent years, a wide variety of mobile and stationary sources of NOx emissions have come under increased scrutiny and regulation.
The rate at which NOx is formed is dependent upon the following variables: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of NOx increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less NOx than the same mixture at a lower temperature, over a longer period of time.
One strategy for achieving lower NOx emission levels is to install a NOx reduction catalyst to treat the furnace exhaust stream. This strategy, known as Selective Catalytic Reduction (SCR), is very costly and, although it can be effective in meeting more stringent regulations, represents a less desirable alternative to improvements in burner design.
Burners used in large industrial furnaces may use either liquid or gaseous fuel. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and mix combustion air with the fuel at the zone of combustion.
Gas fired burners can be classified as either premix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.
Raw gas burners inject fuel directly into the air stream, such that the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763, which patent is incorporated herein by reference. In addition, many raw gas burners produce luminous flames.
Premix burners mix the fuel with some or all of the combustion air prior to combustion. Since premixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Premixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, premix burners are often compatible with various steam cracking furnace configurations.
Floor-fired premix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Therefore, a premix burner is the burner of choice for such furnaces. Premix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.
In gas fired industrial furnaces, NOx is formed by the oxidation of nitrogen drawn into the burner with the combustion air stream. The formation of NOx is widely believed to occur primarily in regions of the flame where there exist both high temperatures and an abundance of oxygen. Since ethylene furnaces are amongst the highest temperature furnaces used in the hydrocarbon processing industry, the natural tendency of burners in these furnaces is to produce high levels of NOx emissions.
The majority of recent low NOx burners for gas-fired industrial furnaces are based on the use of multiple fuel jets in a single burner. Such burners may employ fuel staging, flue-gas recirculation, or a combination of both. U.S. Pat. Nos. 5,098,282 and 6,007,325 disclose burners using a combination of fuel staging and flue-gas recirculation. Certain burners may have as many as 8-12 fuel nozzles in a single burner. The large number of fuel nozzles requires the use of very small diameter nozzles. In addition, the fuel nozzles of such burners are generally exposed to the high temperature flue-gas in the firebox.
In the high temperature environment of steam-cracking furnaces used for the manufacture of ethylene, the combination of small diameter fuel nozzles and exposure to high temperature flue gas can lead to fouling and potential plugging of the fuel jets. This not only has an adverse impact on burner performance, but also increases the cost of maintenance associated with repeated cleaning of fuel nozzles.
However, one advantage of the staged-air pre-mix burners disclosed in U.S. Pat. Nos. 4,629,413 and 5,092,761 (and discussed below) relates to their use of a single fuel nozzle. This permits the size of the fuel nozzle to be the maximum possible for a given burner firing duty. In addition, since the fuel nozzle is located at the inlet to the venturi, it is not exposed directly to either the high temperature flue-gas or the radiant heat of the firebox. For these reasons the problems of fuel nozzle fouling are minimized, providing a significant advantage for the staged-air pre-mix burner in ethylene furnace service.
One technique for reducing NOx that has become widely accepted in industry is known as staging. With staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NOx formation than an air-fuel fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature dramatically reduce NOx emissions. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase.
In the context of premix burners, the term primary air refers to the air premixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air is more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
U.S. Pat. No. 4,629,413 discloses a low NOx premix burner and discusses the advantages of premix burners and methods to reduce NOx emissions. The premix burner of U.S. Pat. No. 4,629,413 lowers NOx emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The manner in which the burner disclosed achieves light off at start-up and its impact on NOx emissions is not addressed. The contents of U.S. Pat. No. 4,629,413 are incorporated by reference in their entirety.
U.S. Pat. No. 2,813,578, the contents of which are incorporated by reference in their entirety, discloses a heavy liquid fuel burner, which mixes the fuel with steam for inspiration prior to combustion. The inspirating effect of the fuel and steam draws hot furnace gases into a duct and into the burner block to aid in heating the burner block and the fuel and steam passing through a bore in the block. This arrangement is disclosed as being effective to vaporize liquid fuel and reduce coke deposits on the burner block and also to prevent any dripping of the oil.
U.S. Pat. No. 2,918,117 discloses a heavy liquid fuel burner, which includes a venturi to draw products of combustion into the primary air to heat the incoming air stream to therefore completely vaporize the fuel.
U.S. Pat. No. 4,004,875, the contents of which are incorporated by reference in their entirety, discloses a low NOx burner, in which combusted fuel and air is cooled and recirculated back into the combustion zone. The recirculated combusted fuel and air is formed in a zone with a deficiency of air.
U.S. Pat. No. 4,230,445, the contents of which are incorporated by reference in their entirety, discloses a fluid fuel burner that reduces NOx emissions by supplying a flue gas/air mixture through several passages. Flue gas is drawn from the combustion chamber through the use of a blower.
U.S. Pat. No. 4,575,332, the contents of which are incorporated by reference in their entirety, discloses a burner having both oil and gas burner lances, in which NOx emissions are reduced by discontinuously mixing combustion air into the oil or gas flame to decelerate combustion and lower the temperature of the flame.
U.S. Pat. No. 5,263,849 discloses a burner system for a furnace combustion chamber having an ignition chamber for discharging an ignited combustible mixture of primary air and fuel into the furnace combustion chamber, and a plurality of nozzle ports for directing a high velocity stream of secondary air into the furnace combustion chamber. The system includes a fuel supply and separately controlled primary and secondary air supply lines. U.S. Pat. No. 5,263,849 discloses the use of an igniter that projects angularly into a flame holder. The contents of U.S. Pat. No. 5,263,849 are incorporated by reference in their entirety.
U.S. Pat. No. 4,708,638, the contents of which are incorporated by reference in their entirety, discloses a fluid fuel burner in which NOx emissions are reduced by lowering the flame temperature. A venturi in a combustion air supply passage, upstream of a swirler, induces the flow of flue gas into the combustion air supply passage from ducts opening into the furnace. A swirler is located at the free end of a fuel pipe and mixes the flue gas with the primary combustion air.
U.S. Pat. No. 5,269,679 discloses a gas-fired burner incorporating an air driven jet pump for mixing air, fuel and recirculated flue gas. The burner is configured for the staged introduction of combustion air to provide a fuel-rich combustion zone and a fuel-lean combustion zone. A pilot flame is provided through a tube that ignites the air and fuel mixture in a diffuser. Combustion can be observed through a scanner tube. The burner is said to achieve reduced NOx emission levels in high temperature applications that use preheated combustion air. The contents of U.S. Pat. No. 5,269,679 are incorporated by reference in their entirety.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NOx emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Airflow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The flue gas recirculating system may be retrofitted into existing burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
Thus, one set of techniques achieves lower flame temperatures by using staged-air or staged-fuel burners to lower flame temperatures by carrying out the initial combustion at far from stoichiometric conditions (either fuel-rich or air-rich) and adding the remaining air or fuel only after the flame has radiated some heat away to the fluid being heated in the furnace.
Another set of techniques achieves lower flame temperatures by diluting the fuel-air mixture with diluent material. Flue-gas (the products of the combustion reaction) or steam is commonly used as a diluent. Such burners are classified as FGR (flue-gas-recirculation) or steam-injected, respectively.
A drawback of the system of U.S. Pat. No. 5,092,761 is that the staged-air used to cool the FGR duct must first enter the furnace firebox, traverse a short distance across the floor and then enter the FGR duct. During this passage, the staged air is exposed to radiation from the hot flue-gas in the firebox. Analyses of experimental data from burner tests suggest that the staged-air may be as hot as 700° F. when it enters the FGR duct.
From the standpoint of NOx production, another drawback associated with the burner of U.S. Pat. No. 5,092,761 relates to the configuration of the lighting chamber, necessary for achieving burner light off. The design of this lighting chamber, while effective for achieving light off, has been found to be a localized source of high NOx production during operation. Other burner designs possess a similar potential for localized high NOx production, since similar configurations are known to exist for other burner designs, some of which have been described hereinabove.
Analysis of burners of the type described in U.S. Pat. No. 5,092,761 has indicated the flue-gas-recirculation (FGR) ratio is generally in the range 5-10% where FGR ratio is defined as:FGR ratio (%)=100[G/(F+A)]where                G=Flue-gas drawn into venturi, (lb)        F=Fuel combusted in burner, (lb), and        A=Air drawn into burner, (lb).        
The ability of these burners to generate higher FGR ratios is limited by the inspirating capacity of the fuel orifice/gas spud/venturi combination. Further closing or partially closing the primary air dampers will produce lower pressures in the primary air chamber and thus enable increased FGR ratios. However, when the ratio of FGR is increased, the flame becomes more susceptible to entrainment into the FGR duct, which raises combustion temperature, which, in turn raises NOx and may cause damage to metal parts.
As disclosed in “The Design of Jet Pumps” by A. E. Knoll, appearing in Vol. 43 of Chemical Engineering Progress, published by the American Institute of Chemical Engineers (1947), it is known to optimize the operation of venturis used in air and steam operated air movers at relatively mild (roughly ambient) temperatures. In contrast, in the burner of the invention, combustible gaseous fuel (including, but not limited to, methane, H2, ethane and propane) is used to move a combination of very hot (above 1000° F.) flue gases, hot air, hot uncombusted fuel (CO), and ambient air.
Additionally, commercial experience and modeling have shown when flue gas recirculation rates are raised, there is a tendency of the flame to be drawn into the FGR duct. Often, it is this phenomenon that constrains the amount of flue gas recirculation. When the flame enters directly into the flue gas recirculation duct, the temperature of the burner venturi tends to rise, which raises flame speed and causes the recirculated flue gas to be less effective in reducing NOx. From an operability perspective, the flue gas recirculation rate needs to be lowered to keep the flame out of the FGR duct to preserve the life of the metallic FGR duct.
Moreover, internal flow dynamics in the area of the fuel riser/burner spud assembly/venturi combination can affect the inspirating capacity of the combination, reducing the ability to achieve FGR ratios in excess of 10%. In this regard, the fuel riser/burner spud centering plate assembly used in some burners to assure that the fuel riser/burner spud assembly is aligned with the venturi, can serve to negatively affect internal flow dynamics in the area of the fuel riser/burner spud assembly/venturi combination, reducing inspirating capacity.
An additional challenge to the designer of low NOx burners is to maintain adequate flame stability. The very techniques used to minimize NOx emissions reduce flame temperature and flame speed, and generally lead to less stable flames, which are more prone to “lift-off.” “Lift-off” is a term used to describe a flame where the point of combustion has left the burner tip. In extreme cases, lift-off can lead to instances of flameout; where combustion at the burner is extinguished. Such a condition is highly undesirable as it can potentially lead to an accumulation of an air/fuel mixture in the firebox.
From the standpoint of NOx production, a drawback has been discovered which is associated with the burner tip of the burner of U.S. Pat. No. 5,092,761. One drawback relates to the inability to precisely distribute air flow adjacent to the burner tip, which can result in localized sources of high NOx production.
Therefore, what is needed is a burner for the combustion of fuel that enables higher flue gas recirculation ratios (FGR) to be utilized while minimizing or eliminating any accompanying reduction in burner stability, yielding reductions in NOx emissions. It is also desirable that the improved burner have the characteristic that localized sources of NOx production are substantially reduced and that lower FGR duct temperatures are achieved, yielding further reductions in NOx emissions.